The quinhydrone electrode may be used to measure the hydrogen ion concentration ( pH) of a solution containing an acidic substance.Bates, Roger G. ''Determination of pH: theory and practice''. Wiley, 1973, pp 246-252

Principles and operation

Quinones form a quinhydrone species via hydrogen bonding between ρ-quinone and ρ-hydroquinone. An equimolar mixture of ρ-quinones and ρ-hydroquinone in contact with an inert metallic electrode, such as

antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...

, forms a quinhydrone electrode. Such devices can be used to measure the pH of solutions. Quinhydrone electrodes provide fast response times and high accuracy. However, it can only measure pH in the range of 1 to 9 and the solution must not contain a strong oxidizing or

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

. A

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...

wire electrode is immersed in a saturated aqueous solution of

quinhydrone 1,4-Benzoquinone, commonly known as ''para''-quinone, is a chemical compound with the formula C6H4O2. In a pure state, it forms bright-yellow crystals with a characteristic irritating odor, resembling that of chlorine, bleach, and hot plastic or ...

, in which there is the following equilibrium : + 2H⁺ +2e⁻. The potential difference between the platinum electrode and a

reference electrode A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each ...

is dependent on the activity,

a_

, of hydrogen ions in the solution. :

E= E^0 + \frac \ln a_

(

Nernst equation In electrochemistry, the Nernst equation is a Thermodynamics#Chemical thermodynamics, chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or electrochemical cell, full cell reacti ...

)

Limitations

The quinhydrone electrode provides an alternative to the most commonly used

glass electrode A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an example ...

. however, it is not reliable above pH 8 (at 298 K) and cannot be used with solutions that contain a strong oxidizing or reducing agent.

References

Electroanalytical chemistry devices Electrodes {{analytical-chem-stub